Minimum-spacing circuit design and layout for PICA

ABSTRACT

PICA test circuits are shown that include a first transistor and a second transistor laid out drain-to-drain, such that a gap between respective drain regions of the first and second transistors has a minimum size allowed by a given fabrication technology.

RELATED APPLICATION INFORMATION

This application is a Continuation application of pending U.S. patentapplication Ser. No. 13/452,092 filed on Apr. 20, 2012, incorporatedherein by reference in its entirety.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.:FA8650-11-C-7105 (National Security Agency). The government has certainrights in this invention.

BACKGROUND

1. Technical Field

The present invention relates to test circuit design, and moreparticularly to creating test circuits for high-resolution picosecondimaging circuit analysis.

2. Description of the Related Art

Picosecond imaging circuit analysis (PICA) is a technique used fortiming measurement and failure analysis of integrated circuits. PICAexploits a side-effect of field effect transistors (FETs) whereby a FETemits a burst of light when its drain region is at a high voltage andits gate transitions from a low voltage to a high voltage. This allowsfor optical imaging of the back side of an integrated chip circuit to,e.g., locate failed transistors and perform other measurements.

In PICA systems, higher resolutions are desirable to ensure good imagingthat can test the limits of circuit design features. To test theresolution of PICA systems, test circuits are created which are designedto produce optical emissions that are close together. Previous attemptsto create such test circuits involved compressing circuit layoutsparallel and perpendicular to FET gates. Compression parallel to thetransistor gate is limited by either polysilicon gate later end-to-endspacing or n-channel metal-oxide-semiconductor to p-channelmetal-oxide-semiconductor spacing. Compression parallel to thetransistor gate is limited by gate pitch. Exemplary spacings accordingto such prior art technologies include 284 nm in the parallel spacingand 220 nm in the perpendicular spacing. As such, previous attempts tocreate PICA test circuits have been limited in their ability to testPICA resolution.

SUMMARY

A circuit includes a first semiconductor device and a secondsemiconductor device which emit light during operation, laid out suchthat light-emitting regions of the respective semiconductor devices areproximal to one another with a gap between the respective light-emittingregions including a target resolution size; and a logic circuitconfigured to accept an input signal and at least one select signal andto bias light-emitting regions of at least one of the respectivesemiconductor to inhibit an optical emission when said at least onesemiconductor device is triggered.

A circuit includes a first transistor and a second transistor laid outdrain-to-drain, such that a gap between respective drain regions of thefirst and second transistors has a minimum size allowed by a givenfabrication technology; a first NOR gate having an output connected tothe drain region of the first transistor and accepting a first selectsignal and an input signal; and a second NOR gate having an outputconnected to the drain region of the second transistor and accepting asecond select signal and the input signal, wherein one of said NOR gatesbiases the connected transistor's drain region, according to the selectsignal of said NOR gate, to inhibit an optical emission when saidconnected transistor is triggered.

A test circuit includes a plurality of test cells arranged in a line.Each test cell includes a first transistor and a second transistor laidout drain-to-drain, such that a gap between respective drain regions ofthe first and second transistors has a minimum size allowed by a givenfabrication technology; a first NOR gate having an output connected tothe drain region of the first transistor and accepting a first selectsignal and an input signal provided by the output of a previous testcell; and a second NOR gate having an output connected to the drainregion of the second transistor and accepting a second select signal andthe input signal. One of said NOR gates in each test cell biases theconnected transistor's drain region, according to the select signal ofsaid NOR gate, to inhibit an optical emission when said connectedtransistor is triggered.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a diagram of a field effect transistor according to thepresent principles;

FIG. 2 is a diagram of an exemplary PICA imaging test circuit accordingto the present principles;

FIG. 3 is a diagram showing an exemplary physical layout of a PICAimaging test circuit on a chip according to the present principles;

FIG. 4 is a diagram showing an exemplary PICA testing apparatusaccording to the present principles;

FIG. 5 is a series of signal graphs showing signal values over time atvarious points within a PICA imaging test circuit;

FIG. 6 is a series of signal graphs showing signal values over time atvarious points within a PICA imaging test circuit;

FIG. 7 is a diagram of a PICA imaging test chip that includes multiplePICA imaging test circuits according to the present principles; and

FIG. 8 is a block/flow diagram showing a method for testing an imagingdevice according the present principles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Picosecond Imaging for Circuit Analysis (PICA) is a powerful opticaltool that permits probing time-resolved emission signals collected fromtransistors inside a circuit, allowing for non-invasive testing ofcircuit operation. By neglecting the arrival time of photons andconstructing a spatial histogram of the measured light, one can create a“PICA image” of the emission intensity. Bright spots indicate individualtransistors or sub-circuit units composed by several transistors. Byselecting a portion of the image, one can create a histogram of thephoton arrival time and, therefore, a waveform in time of the emissionactivity from that location.

Alternatively, 2D images of the photons at specific times can be createdand then a video may be constructed by combining frames. It should benoted that more than one bright spot is usually present in such a PICAimage, and the emission may correspond to gates that are not switching,or which are switched with a certain frequency.

Testing the resolution of a PICA camera means determining whethercircuit features having a given separation can be distinguished. Forexample, a PICA camera that can distinguish between adjacent features100 nm apart has a resolution of at least 100 nm. However, the realitiesof modern circuit fabrication technologies place practical limitationson how small circuit features can be made and how close together theycan be placed.

Field effect transistors (FETs) emit light from their drain regionsduring operation. One partial solution to forming a high-resolution testcircuit is to form two FETs that share a single gate and source node,but have separate drain nodes. This may be used to achievedrain-to-drain spacing that has diffusion edge-to-edge spacing of about70 nm in a 32 nm fabrication technology. 70 nm is near the resolutionlimit of PICA tools, and is therefore an effective design for testingthe resolution of such tools. As such, the present principles providetest circuits that have minimally spaced transistors sharing a commongate and source node. NOR gates are used to drive separate signals onthe drains of the two transistors. Emission from switching gates may bemodulated in time, while non-switching gates either do not have anassociated emission, or their emissions are not modulated in time.

There are several applications which benefit from quickly distinguishingwhich bright spot or spots correspond to a switching gate and which donot. One example relates to debugging electrical patterns of a circuitto quickly identify which gate is exercised by the specific pattern,pattern tuning, and pattern debugging. This also helps in applicationssuch as logic state mapping. Another exemplary application relates tosecurity and detecting undesired chip modifications, for example byidentifying a set of switching gates and their position in the layoutand comparing the identified set to an exemplar.

Being able to detect which gate is switching among many non-switchingidentical gates helps in circuit probing and diagnostics when limitedinformation about schematic, layout, and circuit behavior is availableto a tool operator. Another use for PICA imaging includes assisting atool operator in better defining and optimizing the region of interestfor extracting time resolved waveforms or for further probing withsingle-point detectors. By readily identifying the regions thatcorrespond to switching gates, one can more easily define the border ofthe region of interest.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer; however,other architectures, structures, substrate materials and processfeatures and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip of photovoltaic device may becreated in a graphical computer programming language, and stored in acomputer storage medium (such as a disk, tape, physical hard drive, orvirtual hard drive such as in a storage access network). If the designerdoes not fabricate chips or the photolithographic masks used tofabricate chips, the designer may transmit the resulting design byphysical means (e.g., by providing a copy of the storage medium storingthe design) or electronically (e.g., through the Internet) to suchentities, directly or indirectly. The stored design is then convertedinto the appropriate format (e.g., GDSII) for the fabrication ofphotolithographic masks, which typically include multiple copies of thechip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, an exemplary embodiment ofa FET according to the present principles is shown. Although only FETsare shown herein, it is contemplated that any semiconductor device canbe used in accordance with the present principles if said deviceproduces optical emissions during operation, A substrate layer 102 isformed from any suitable bulk material including, e.g., silicon. Adielectric layer 104 on the substrate layer 102 is formed from anysuitable dielectric material such as, e.g., silicon dioxide. An activelayer is formed on the dielectric layer 104 including a source region106, a channel region 108, and a drain region 110, and may be formedfrom, e.g., doped silicon. The doping the channel region 108 may be ofthe same kind as the source region 106 and drain region 110, or may beof the opposite polarity.

An insulating layer 111 is disposed on the source 106, channel 108, anddrain 110 and may be formed from, e.g., any suitable dielectric. Theinsulating layer 111 has a source contact 112 and a drain contact 114that run through the layer 111, providing electrical access to thesource region 106 and drain region 110. A gate electrode 116 runsthrough the insulating layer 111 and is separated from the channelregion 108 by a gate dielectric 118. It should be stressed that thedepicted FET design is just one that may be used according to thepresent principles. There are a wide variety of FET designs in the art,and it is contemplated that any appropriate FET can be used.

A FET such as that shown in FIG. 1 will produce an optical signal when avoltage applied to the gate electrode 116 rises above a thresholdvoltage and voltage at the drain 110 is high, before starting to fall.Thus, if the voltage at the drain 110 is kept biased at a low voltage(e.g., ground), an FET will not emit light when the gate 116 istriggered. This permits selective suppression of FET emissions by usinga control signal to bias the FET drain 110 as desired.

Referring now to FIG. 2, a test buffer circuit 200 is shown. The circuitincludes two FETs 212 and 214 triggered at gate electrode 116 by acommon input signal 206, where said input signal may be provided as theoutput signal of a previous buffer circuit 200. The FETs 212 and 214 areconnected by their respective source terminals 106 to a common ground216. The drain terminals 110 of the FETs 212 and 214 are eachrespectively connected to the output of NOR gates 202 and 204,represented by a first node 222 and a second node 224. The NOR gates 202and 204 each receive a common input 206 and a respective select input,either sel-1 208 or sel-2 210. The outputs of both NOR gates 202 and 204are used as the inputs to a third NOR gate 218, which provides itsoutput to inverter 220.

The combination of a NOR gate 202 and FET 212 as shown essentially formsa three-input NOR gate. The select signals sel-1 208 and sel-2 210 areprovided externally as opposing waveforms, such that sel-1 208 will havea high value when sel-2 210 has a low value. The select signals 208 and210 may therefore be switched at regular intervals according to adesired test pattern. For example, the select signals 208 and 210 mayalternate values at a rate based on the clock rate of the input signal206, to allow a desired number of optical pulses to be emitted from eachtransistor 212 or 214 before switching. Alternatively, select signals208 and 210 may be held static to allow sufficient light to collect toproduce a detectable signal.

If select signal sel-1 208 is low (i.e., logical “0”), then theoscillating input signal 206 will cause the logical value of the signalat node 222 to oscillate with a value opposite that of the input signal206. As a result, the first transistor 212 emits flashes of light at thedrain 110, while the second transistor 214 does not. This is becauseselect signal sel-2 210 causes the drain 110 of transistor 214 to bebiased to a logical 0 by the output of the second NOR gate 204, suchthat triggering the second transistor 214 does not cause a voltagechange. Similarly, if the second select signal sel-2 210 is low, thennode 224 will oscillate with a value opposite that of the input signal206 and node 222 will remain fixed at logical 0. In such a case, thesecond transistor 214 emits a flash of light while the first transistor212 remains quiescent.

The output of each transistor-NOR pair, represented by nodes 222 and224, are then passed through a third NOR gate 218 and inverted atinverter 220. This makes the overall function of the circuit 200 that ofa buffer, where the output of inverter 220 is the same as the inputsignal 206.

It should be noted that the NOR gates 202 and 204 represent onepreferred embodiment of the present invention, but could in practice bereplaced by any logic circuit that accepts the input 206, the two selectsignals 202 and 204, and produces biased outputs 222 and 224 accordingto the input states. The NOR-gate arrangement described herein providesa particularly efficient and compact embodiment, but other combinationsof logic gates could be used instead.

Using the above circuit 200, the select signals 208 and 210 may beswitched between 0 and 1, causing optical emissions detected by a PICAtool to “jump” spatially from the drain 110 of transistor 212 to that oftransistor 214. By using a common gate input 116 the physical spacingbetween transistors 212 and 214 may be minimized, allowing forhigh-resolution PICA testing. The select signals 208 and 210 may beswitched at any appropriate rate, including for example the chip's clockrate and a refresh rate for the PICA imaging device.

Referring now to FIG. 3, a top-down view of the circuit 200 is shown assaid circuit could be laid out on a chip. Component interconnections areomitted for clarity. As can be seen, the transistors 212 and 214 arerelatively large devices, such that the optical signal produced uponswitching is more substantial. The transistors 212 and 214 are formeddrain-to-drain with a spacing 302 between them. This spacing 302represents the physical quantity that the present principles provide fortesting PICA camera resolutions. An exemplary spacing has been shown ofabout 70 nm using 32 nm technology, but because the spacing 302 betweentransistors 212 and 214 is not limited by a polysilicon gate pitch or byPFET-to-NFET spacing, the spacing can be made as small as the minimumallowed by the fabrication technology used. As new fabricationtechnologies are devised, the present principles may be applied toreduce the spacing between drains 110 of gates 212 and 214 even further.Furthermore, the spacing can easily be increased above said minimum toallow evaluation of PICA tools that cannot resolve the minimum spacing.This can be accomplished by creating a layout which places thetransistors 212 and 214 farther apart.

Referring now to FIG. 4, a diagram of an exemplary testing setup isshown. While it is contemplated that the present principles may employedin any imaging system to test maximum resolution, a PICA system is shownin particular. A PICA imager 402 scans a test circuit 404 to detectlight emissions from FETs. The PICA imager 402 may take a picture of theentire test circuit at once or it may capture information in apixel-at-a-time fashion. The PICA imager 402 then stores imaginginformation in storage 406, which may be any appropriate form ofstorage, including a disk, tape, physical hard drive, or virtual harddrive such as in a storage access network. The PICA imager 402 may beself-controlled, or may be controlled by a test generator 408. The testgenerator 408 provides pattern data to test circuit according to apredetermined test sequence. For example, the test generator 408 maycontrol select signals 208 and 210 to produce switching patterns betweenpaired transistors 212 and 214. The test generator may also controlphysical scanning if needed, for example if the PICA imager 402 or testcircuit 404 moves with respect to the other for scanning and imagingpurposes.

When the PICA imager 402 scans the test circuit 404, it builds a seriesof images in storage 406. These images are then analyzed to determinewhether the PICA imager 402 has met resolution requirements. Inparticular, it is considered whether the PICA imager 402 is able todistinguish between the light emissions from a first transistor 212 anda second transistor 214. In some cases, the PICA imager 402 may not besensitive to detect the output of a single transistor. In that case, thecircuit 200 may be repeated and chained, with the output of inverter 220forming the input 206 of the next circuit 200. By lining the circuits200 vertically, a strip of active transistors can be created and moreeasily detected by PICA imager 402. It should also be noted that theexposure length of PICA imager 402 may be orders of magnitude longerthan the clock cycle of the input signal 206. As such, the selectsignals 208 and 210 may be alternated at a rate of once per exposure,rather than being based on the clock rate of the input signal 206.

If a single pixel of recorded light information covers the emissionsfrom both transistors, then the PICA imager 402 has a resolution lowerthan that needed to fully capture the emission information from the testcircuit 404. However, if the PICA imager 402 can reliably distinguishbetween the emissions from the neighboring transistors 212 and 214, thenthe imager 402 meets or exceeds the resolution range tested by the testcircuit 404.

Referring now to FIG. 5, logical values for signals at a series ofpoints in the circuit of FIG. 2, assuming a logical “0” on the firstselect signal 208 and a logical “1” on the second select signal 210. Thehorizontal axis on each graph represents time, while the vertical graphrepresents the logical value of the signal. It is specificallycontemplated that the value may, in turn, represent the voltage of thesignal, but the renderings have been kept at a qualitative level forclarity. The input signal 206 oscillates between a logical 1 and alogical 0. The input signal is illustratively described as being adigital square wave, though it is contemplated that any appropriateinput signal may be employed. With sel-1 208 being fixed at logical 0,the first NOR gate 202 outputs to node 222 an inversion of the inputsignal 206. With sel-2 210 being fixed at logical 1, the second NOR gate204 outputs a fixed logical 0. Because FET optical emissions occur ifthe gate electrode 116 is triggered while voltage is high at drain 110,and because a low-voltage at 224 biases the drain of the secondtransistor 214, the second transistor will not emit light while sel-2210 is set to logical 1. In contrast, the first transistor 212 willflash in time with the input signal 206.

Referring now to FIG. 6, logical values for signals at a series ofpoints in the circuit of FIG. 2, assuming a logical “1” on the firstselect signal 208 and a logical “0” on the second select signal210—inputs that are reversed from those shown in FIG. 5. Again, thehorizontal axis on each graph represents time, while the vertical graphrepresents the logical value of the signal. The input signal 206continues to oscillate between a logical 1 and a logical 0. With sel-1208 being fixed at logical 1, the first NOR gate 202 outputs a fixedlogical 0 to node 222. With sel-2 210 being fixed at logical 0, thesecond NOR gate 204 outputs an inversion of the input signal 206 to node224. Thus, because a low-voltage at 222 biases the drain of the firsttransistor 212, the first transistor will not emit light while sel-1 208is set to logical 1. In contrast, the second transistor 214 will flashin time with the input signal 206.

Thus the select signals 208 and 210 control which of transistors 212 and214 will emit light. The select signals 208 and 210 may then bealternated to cause the light emissions to “jump” between thetransistors, providing a predictable signal for the PICA imager 402 todetect.

Referring now to FIG. 7, an exemplary layout for a testing chip 700 isshown. The chip 700 includes multiple banks 706 and 708 of individualtest circuits 200. The test circuits 200 are aligned such that an entirerow of transistors will activate at once. This increases the amount oflight output along the bank, such that the resolution of a PICA imager402 can be more easily tested. The banks 706 and 708 receive inputs frominput generator 702 and select generator 704. These signals may begenerated on-chip or they may be provided off-chip by, e.g., testgenerator 408. The banks 706 and 708 may furthermore be linked to oneanother, such that the output the last test circuit 200 in bank 706produces the input for the first test circuit 200 in bank 708.

The circuit layout described in FIGS. 2 and 3 has additionalapplicability in minimizing device mismatch. Device mismatch is acondition where transistor characteristics (e.g., threshold voltage)vary across a single chip. Wafer manufacturing processes may causeundesirable variations in transistor characteristics across the wafersuch that, for example, components on the left side of the wafer mighthave a higher threshold voltage than components on the right side of thewafer. The present principles may therefore be applied to a differentialpair of transistors where having matched characteristics is desirable.The present principles allow for the placement of transistors as closetogether as possible to avoid the negative effects of device mismatch.This may have particular applicability where precise timing isimportant, because a difference in threshold voltage may cause a FET totrigger sooner or later than intended.

Referring now to FIG. 8, a method for testing an imaging deviceaccording to the present principles is shown. Block 802 forms a set ofsemiconductor devices such that their light-emitting regions are next toone another. This could be in the layout shown in FIG. 7, or could takeany other appropriate layout. Block 804 forms logic circuits to controlthe semiconductor devices. Again, these could be the NOR gates 202 and204 described above, or could be any other suitable combination of logiccircuits adapted to control the light emissions of the semiconductordevices. Block 806 provides an input signal 206 to activate thesemiconductor devices, and block 808 provides select signals 208 and 210to the logic circuits to selectively suppress light emissions from someof the semiconductor devices. The select signals are chosen such that apredictable pattern of light emissions is produced. Block 810 thenchanges the select signals to change the pattern of light emission.Block 812 determines whether an imaging device, such as PICA camera 402can discern between the patterns of light emission. If so, the imagingdevice has sufficient resolution of at least the spacing between thesemiconductor devices.

Having described preferred embodiments of a minimum-spacing circuitdesign and layout for PICA (which are intended to be illustrative andnot limiting), it is noted that modifications and variations can be madeby persons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A circuit, comprising: a first semiconductordevice and a second semiconductor device which emit light duringoperation, laid out such that light-emitting regions of the respectivesemiconductor devices are side-by-side with a gap between the respectivelight-emitting regions including a target resolution size.
 2. Thecircuit of claim 1, further comprising a logic circuit configured tobias light-emitting regions of at least one of the respectivesemiconductor to inhibit an optical emission when said at least onesemiconductor device is triggered.
 3. The circuit of claim 2, whereinthe logic circuit comprises two NOR gates, each accepting respectiveselect signals that alternate logical values.
 4. The circuit of claim 2,wherein the logic circuit is configured to bias a drain region of afield effect transistor at a low voltage to inhibit light emission.
 5. Acircuit, comprising: a first transistor and a second transistor laid outsuch that a drain of the first transistor faces a drain of the secondtransistor, such that a gap between respective drain regions of thefirst and second transistors has a minimum size allowed by a givenfabrication technology.
 6. The circuit of claim 5, wherein the gapbetween the respective drain regions has a size of about 70 nm or less.7. A test circuit, comprising: a plurality of test cells arranged in aline, each test cell comprising a first transistor and a secondtransistor laid out drain-to-drain, such that a gap between respectivedrain regions of the first and second transistors has a minimum sizeallowed by a given fabrication technology.
 8. The test circuit of claim7, wherein adjacent test cells are oriented with respect to one anothersuch that the first transistors of the test cells form a first line andthe second transistors of the test cells form a second line.
 9. Acircuit, comprising: a first semiconductor device and a secondsemiconductor device which emit light during operation, laid out suchthat light-emitting regions of the respective semiconductor devices areproximal to one another with a gap between the respective light-emittingregions including a target resolution size; and a logic circuitconfigured to accept an input signal and at least one select signal andto bias light-emitting regions of at least one of the respectivesemiconductor to inhibit an optical emission when said at least onesemiconductor device is triggered.
 10. The circuit of claim 9, whereinthe first and second semiconductor devices are field effect transistors.11. The circuit of claim 9, wherein the logic circuit comprises two NORgates, each accepting respective select signals that alternate logicalvalues.
 12. The circuit of claim 9, wherein the at least one selectsignal comprises two select signals having opposite logical values. 13.The circuit of claim 9, wherein the semiconductor devices are fieldeffect transistors.
 14. The circuit of claim 13, wherein the logiccircuit is configured to bias a drain region of a field effecttransistor at a low voltage to inhibit light emission.
 15. A circuit,comprising: a first transistor and a second transistor laid outdrain-to-drain, such that a gap between respective drain regions of thefirst and second transistors has a minimum size allowed by a givenfabrication technology; a first NOR gate having an output connected tothe drain region of the first transistor and accepting a first selectsignal and an input signal; and a second NOR gate having an outputconnected to the drain region of the second transistor and accepting asecond select signal and the input signal, wherein one of said NOR gatesbiases the connected transistor's drain region, according to the selectsignal of said NOR gate, to inhibit an optical emission when saidconnected transistor is triggered.
 16. The circuit of claim 15, furthercomprising: a third NOR gate accepting the outputs of the first andsecond NOR gates; and an inverter accepting the output of the third NORgate.
 17. The circuit of claim 16, wherein the inverter has an outputthat conforms to the input signal.
 18. The circuit of claim 15, whereinthe first select signal and the second select signal have oppositelogical values.
 19. The circuit of claim 15, wherein the first selectsignal and the second select signal alternate logical values.
 20. Thecircuit of claim 19, wherein the first select signal and the secondselect signal alternate logical values at a periodic interval.
 21. Thecircuit of claim 15, wherein the input signal is a square wave.
 22. Thecircuit of claim 15, wherein the gap between the respective drainregions has a size of about 70 nm or less.
 23. A test circuit,comprising: a plurality of test cells arranged in a line, each test cellcomprising: a first transistor and a second transistor laid outdrain-to-drain, such that a gap between respective drain regions of thefirst and second transistors has a minimum size allowed by a givenfabrication technology; a first NOR gate having an output connected tothe drain region of the first transistor and accepting a first selectsignal and an input signal provided by the output of a previous testcell; and a second NOR gate having an output connected to the drainregion of the second transistor and accepting a second select signal andthe input signal, wherein one of said NOR gates in each test cell biasesthe connected transistor's drain region, according to the select signalof said NOR gate, to inhibit an optical emission when said connectedtransistor is triggered.
 24. The test circuit of claim 23, whereinadjacent test cells are oriented with respect to one another such thatthe first transistors of the test cells form a first line and the secondtransistors of the test cells form a second line.
 25. The test circuitof claim 24, wherein the first and second select signals are chosen toprovide a line of illumination for optical testing.
 26. The test circuitof claim 23, wherein each test cell further comprises: a third NOR gateaccepting the outputs of the first and second NOR gates; and an inverteraccepting the output of the third NOR gate and producing an output forthe test cell.
 27. The test circuit of claim 26, wherein the inverterhas an output that conforms to the input signal.
 28. The test circuit ofclaim 23, wherein the first select signal and the second select signalhave opposite logical values.
 29. The test circuit of claim 28, whereinthe first select signal and the second select signal alternate logicalvalues at a periodic interval.